This invention relates in general to circulating fans and in particular to mixed flow fans of the axial fan configuration. More specifically, but without restriction to the particular use which is shown and described, this invention relates to mixed flow fans of the axial fan configuration having rotating vaneless diffusers attached thereto.
Circulating fans are found in many diverse applications ranging from the common household cooling fan to the more sophisticated jet engine compressor fan. Other applications include fans designed for use in refrigerator and freezer compartments, cooling fans for lighting fixtures, and circulating fans for use in recreational rooms, bathrooms, closets and the like. Although the invention disclosed herein has general application as a mixed flow fan, for convenience of illustration, the preferred embodiment will be described with reference to a mixed flow fan and rotating vaneless diffuser incorporated in a ceiling mounted, illuminated ventilating unit. An example of such a vaneless diffuser is disclosed in my copending application, Ser. No. 57,592 for Ductless Air Treating Device with Illuminator filed concurrently herewith.
Fans may be classified generally as one of four types, axial, centrifugal, transverse, or mixed. An axial fan takes air in axially from a direction parallel to the axis of the fan rotation, and also discharges the air in the axial direction. A centrifugal fan also takes in air axially, but discharges the air radially in a direction perpendicular to its axis of rotation. A mixed flow fan takes in air axially and discharges the air in a direction lying between the axial and radial directions. Other considerations involved in the design and construction of air circulating or ventilating devices include the configurations of the blades and the housing enclosure.
The blade is constructed in accordance with the following range of applicable dimensions, which are illustrated graphically in diagram A. The dimension c is the blade arc length measured between the leading and trailing edges of the blade. R is the radius of curvature of the blade, l is the cord length between the leading and trailing edges of the blade, and the angle is the angle between the cord and radius R of the blade. The distance f is known as the camber, and the angle is the pitch angle of the blade. The total axial depth of the blade is shown by the letter B. The angles .alpha. and .beta. generally range between 40.degree. and 60.degree., and the camber to cord ratio varies between 0.10 and 0.25. The dimensions B, c and R, are established from the relationships:
TABLE I ______________________________________ B = 1 sin .beta. ##STR1## ##STR2## ______________________________________
The desired B dimension is selected to suit the application and is generally in the order of from 15% to 40% of the impeller diameter (2R).
For example, suitable dimensions of a fan designed for refrigerator and freezer evaporator applications would include a 41/2 inch diameter impeller having an axial blade depth of 0.75 inch. By fixing both the angles .alpha. and .beta. at 45.degree., the impeller will have a discharge angle from the trailing edge of each blade parallel to the axis of fan rotation. The significant dimensions therefore are as follows:
.alpha.=B=45.degree. PA1 B=0.75 in. PA1 l=1.0607 in. PA1 R=0.75 in.
A suitable hub diameter for the fan would be 1.25 inches, which would give a hub to tip ratio of 0.2778.
Section of a suitable diffusion housing is essential to the proper operation of a circulating fan. The basic concept behind the operation of a diffuser is the conversion of energy. Referring to diagram b, a simple diffuser is illustrated with the air flowing from left to right in the direction of the arrow. The air in this system may be described mathematically in an energy conservation equation which relates to the velocity pressure of the air, which is analogous to its kinetic energy, and the static pressure of the air, which is analogous to its potential energy. Assuming no energy losses, the total energy level of the air in the system must be constant at both points, and it is evident from the equation that the function of a fan is to convert the velocity pressure into static pressure. The sum of the velocity pressure and the static pressure at point 1 must equal the sum of the velocity pressure and static pressure at point 2.
The configuration for the vane diffuser or scroll for a fan having an impeller radius of R is determined by drawing a line equal in length to the circumference of the impeller 2.pi.R as shown in diagram c. By projecting a line from point A in the diagram outwardly at the selected diffusion angle 2.gamma. and rolling the line back up in a circle as is shown in diagram d the general shape of the housing is determined.
For maximum efficiency and diffusion rate, a 14.degree. angle of diffusion (2.gamma.) for the housing or scroll is preferred. However, from practical considerations, housings having this diffusion angle are generally too large. Therefore, as a tradeoff, smaller diffusion angles, for example 5.degree. to 8.degree., are generally used.
Within a given housing a fan is capable of developing different levels of static pressure depending upon the blockage to air flow. Maximum pressure occurs when complete blockage of the discharge is effected. Maximum flow or free flow occurs when no restriction to the discharge is imposed. The different points or levels of energy between complete blockage and free flow comprise what is known as the air performance capability of a fan. The performance curve for a mixed flow fan contained within a 6.5 diffusion housing is shown by the dotted line in diagram e.
The solid line in diagram e depicts the operating characteristics of the same mixed flow fan operating in a static vaneless diffuser without the 6.5 diffusion housing. A significant gain in performance, an increase of approximately 30% in the free flow air delivery rate, is realized. This gain may be translated into a speed reduction of 30% which, in turn, results in substantially lower noise levels. A further reduction of the rotational noise level may be obtained by attaching the vaneless diffuser to the fan itself. An impeller having a rotating vaneless diffuser as an integral part thereof reduces the clearance between the blade tip and the diffuser effectively to zero which eliminates much of the air entry shock losses and improves the volumetric and mechanical efficiency of the device.